Research On Characteristic Of Ground Vibrations Of Debris Flow Based On Geophone And Fiber Bragg Grating Monitoring System

The purpose of this paper is to detect the ground vibration generated by simulation experiments of debris flow by using both geophone system and Fiber Bragg grating sensing system. The Fiber Bragg grating sensing system has lots of advantages in terms of high sensitivity, very low transmission loss, strongly resistance to electromagnetic interference. It makes up the geophone’s shortcomings, which are poor signal quality and easy to decay so that the monitoring range of ground vibration is too small. In this research, the ground vibration was mainly measured by the geophone system, and it was also measured by a complementary sensing system based on fiber optic sensors. The main analysis methods include the Fast Fourier Transform (FFT), Gabor Transform. Moreover, Cross-correlation and accumulative energy method of the ground vibration per unite time were used to calculate the mean velocity of different types of debris flows. The main contents and conclusion were listed follow.1. A ground vibration detector device called geophone, type GS-20DX made by the Geospace Technologies company in the United States is the sensor for forming the geophone system. In addition, the grating acceleration sensor that researched and developed by ourselves independently was used to establish the the Fiber Bragg grating sensing system.2. The simulation experiment scheme of debris flow was designed for producing geo-sound signal. It included experiments of:a free-falling rock hitting the gravel layer; the ground vibration was produced by collisions or friction between different materials of debris flow and the ground; the energy attenuation characteristic of geo-sound and its propagation velocity were investigated.3. The characteristic of ground vibration produced by experiment was discussed, in terms of the characteristic of time domain signal, the peak frequency range, the attenuation characteristic and the propagation velocity. The experimental results indicate that:With the signal measured by the same sensor under the condition of different experimental materials, the time domain analysis showed that the amplitude value of velocity is maximum under the condition of big stone, followed by small stone, silt. The amplitude value of coarse sand and fine sand velocity is minimum. On the other hand, the frequency domain analysis results show that the frequency of the ground vibration in terms of big stone, small rock, silt is in the range of 20Hz-350Hz,20Hz-250Hz, 20Hz-200Hz respectively. However, the main frequency range of coarse sand and fine sand cannot be distinguished. It reflects that coarse sand and fine sand do virtually no contribution to the main frequency of debris flow. The comparison result of time domain signal which in different sensors under the condition of same experimental materials reflects that the attenuation characteristic of energy of geo-sound with distance; Frequency domain analysis indicate:frequency domain graph shape is similarly, and the same main frequency range is similar in different sensors under the same experimental condition. The denser the material of surface is, the slower the attenuation of geo-sound propagation velocity of debris flow becomes; And vice versa.4. We can calculate the average velocity of debris flow under the condition of big stone by using the methods of Cross-correlation and accumulative energy method of the ground vibration per unite time. In addition, geo-sound signal of debris flow measured by geophone was not obvious under the condition of small stone, coarse sand, fine sand and silt. The analysis result show that the average velocity of debris flow was not calculated by the above two methods, because there was no large enough velocity amplitude of geo-sound produced by gravel impact or friction surface.5. The results that monitored by both the geophone system and the Fiber Bragg grating sensing system were compared. The results indicate that Fiber Bragg grating sensing system have higher sensitivity and better signal quality. In addition, the ground vibration signal of debris flows measured by the Fiber Bragg grating sensing system was more obvious. Therefore, the feasibility, which the Fiber Bragg grating sensing system applied to the measurement of geo-sound signal of debris flow had been confirmed. This paper also offered a proposal for improvement and optimization.